This invention relates to a method and an apparatus for monitoring the thickness of an organic material, such as an oil film, on a surface of a substrate, for example on material in a rolling mill, using fluorescence.
The use of fluorescence from an oil film to detect the film or to monitor its thickness is known, for example from U.S. Pat. No. 4,956,558 (Batishko et al.) in which optical fibres are used to monitor fluorescence from lubricating oil illuminated by ultraviolet light; the measurements can be calibrated by observing the fluorescent intensity with films of known thickness. WO 93/22655 (Thiokol Corp.) describes apparatus for detecting contamination such as grease on a surface, using a light beam incident on the surface, and a tunable filter to monitor light reflected or scattered from the surface at a wavelength corresponding to an optical property of the material. One such optical property is fluorescence, and the measurements can be calibrated by inspecting a surface known not to fluoresce, so as to obtain a baseline or zero signal level. In DE 3038107 A (Keck et al.) a fluorescence technique is used to monitor for oil contamination on water, the incident light beam being pulsed and the detector being sensitive to that frequency, so that stray light from other sources does not affect measurements.
In hot metal rolling, whether of ferrous or non-ferrous metals, lubrication by a rolling oil in the bite of the rollers is believed to play an important role in the finish attained. An aim of the invention is to provide a way of monitoring and measuring the thickness of an oil film on a roller as it leaves the bite. Other applications are the measurement of oil film thickness applied to steel sheet in preparation for the automotive industry and the detection of residual oils. A problem with such measurements is that fluorescence intensities depend on the characteristics of the surface of the sheet, such as its reflectivity. This invention provides a scheme for compensating for these quantities.
According to the present invention there is provided a method of monitoring the thickness of oil on the surface of a metal substrate, the method comprising the steps of illuminating a region of the surface with radiation of a suitable wavelength to cause fluorescence in the oil, and detecting the intensity of the fluorescent radiation, characterized by also illuminating the same region of the surface at the same time with a reference beam that experiences negligible absorption when incident on the oil, and measuring the intensity of the reflected reference beam to provide a measure of the reflectivity of the surface, and determining the thickness of the oil at that region from both the values of the fluorescent intensity and the reflectivity of the surface.
The invention also provides such a method, characterized by also measuring the intensity of radiation reflected from the same region of the surface to provide a measure of the reflectivity of the surface, and determining the thickness of the oil at that region from both the values of the fluorecent intensity and the reflectivity of the surface, wherein the intensity of the fluorescent radiation is measured at two different wavelengths, in order to enable changes of temperature to be taken into account.
The invention also provides an apparatus for performing such a method.
The source of radiation to cause fluorescence desirably produces ultraviolet light, and the preferred source is a pulsed xenon flash lamp. This is desirably combined with a filter so the surface is only illuminated with suitable u-v wavelengths to cause fluorescence. Where there is a reference beam, it is preferably incident substantially normally to the surface.
For detecting the reflected radiation the preferred detector is a photodiode. In a preferred arrangement the intensities of both the reflected reference beam and the reflected fluorescence-causing radiation are detected. To detect the latter a photodiode is desirably combined with a filter similar to that used in the source.
The fluorescent radiation is of much lower intensity and may be detected by a photomultiplier or an intensified diode array, desirably combined with a filter to select only the desired fluorescent wavelengths. This filter might be replaced by a monochromator. If the radiation source is pulsed, then the fluorescent radiation detector is desirably gated in synchronisation with the pulses, so that the effect of any ambient light is suppressed.
The reflectivity of the surface of the substrate affects the intensity of the fluorescent light in two ways: a highly reflective surface reflects the incident radiation back through the organic layer, causing more fluorescence, and also reflects some of the fluorescent radiation that was initially travelling away from the collection optics. Measuring the reflected radiation enables these effects to be taken into account.